Stents are utilized in a wide range of surgical procedures. For example, stents are used to repair and support injured tissue during the subsequent healing process. The stent is delivered to the target site and expanded to several times its original diameter until the stent contacts the surrounding tissue. This process is known as initial expansion of the stent. Next, the stent is further expanded to imbed the stent into the walls of the surrounding anatomical structure, for example, an artery. This process is known as imbedding. The process of initial expansion and imbedding is known as deployment. Once the stent is expanded, it takes on a permanent set.
An example of a common surgical procedure involving the use of a stent is the placement of a stent within a coronary artery after removal of plaque from within the artery. In that case, the stent is used to support the vessel which has been blocked by atherosclerotic plaque.
Stents are also used in surgical procedures involving the ureters or the urethra. For example, in prostate surgery, stents are used to hold open tracts of the urinary system.
The placement and positioning of the stent is crucial during all surgical procedures such as those described above. Thus, various procedures have been developed which allow a physician or surgeon to view a stent in situ for proper placement. The most common of these procedures is to view the stent using a fluoroscope.
Prior art stents are commonly made of a single material such as stainless steel, tantalum, or Nitinol.TM., with the most common material used being stainless steel. A major disadvantage of a stainless steel stent is that it is transparent to a fluoroscope. Therefore, using a stainless steel stent requires that opaque dyes be injected in the bloodstream to make the stent visible to the surgeon for positioning and deployment. These dyes dissipate very quickly, making the stent visible for only a brief period of time. Thus, procedures involving stainless steel stents and the use of dye to view the stent require rapid placement and deployment of the stent. Additionally, the lack of visibility of the stent makes it extremely difficult, if not impossible, to verify that the stent has not changed location over time.
Tantalum is a radiopaque material widely used in stents. A solid tantalum stent must have a minimum thickness to be useful in deployment and function. The required thickness of the solid tantalum stent results in a high luminosity on a fluoroscope, and in turn causes several problems. One is that the fluoroscope image produced by tantalum stents is so luminous that it obliterates the detail of the stent pattern and the detail at the stent/vessel interface. Because it is impossible to view the stent/vessel interface, accurate placement of the stent at vessel bifurcations is tedious. Moreover, the fact that the stent structure cannot be accurately observed makes more difficult the determination of whether vascular conditions, such as restenosis, have occurred at the stent site.
Because stents are used at various anatomical sites, it is necessary to vary the thickness, and therefore the strength of the stent, to compensate for anatomical variations. For example, stents may be used at anatomical sites having varying degrees of muscle mass. A multilayer stent would have to be able to compensate for variations in muscle mass, for example, with different thicknesses of the stent layers. Additionally, it would also be desirable to be able to vary the luminosity of a stent to compensate for different anatomical variations and varying degrees of muscle mass.
Surgical stents undergo tremendous plastic deformation during deployment. In a multilayer stent, the layers of the stent must not delaminate or separate. Any delamination of a multilayer stent could expose rough or jagged edges, leading to thrombosis, and direct anatomical injury, including the tearing of vessels.
It is a usual surgical practice to deploy, expand, and imbed surgical stents by means of a balloon unit. The use of these balloon units is well known in the stent art. The pressure employed to expand a surgical stent using a ballon unit is critical. Stents that require a higher pressure to expand run an increased risk of balloon rupture, which could lead to an embolism.
There is, therefore, a need for a stent which is visible on a fluoroscope, but does not mask anatomical structures.
There is also a need for a multilayer stent which can expand without delaminating.
There is also a need for a multilayer stent whose strength can be varied by varying its thickness to accommodate different anatomical structures.
There is further a need for a multilayer stent where its luminosity on a fluoroscope can be varied by changing the thickness of the layers of the stent to accommodate different anatomical structures.
There is still further the need for a multilayer stent where the stent is expanded at a lower pressure and which provides a decreased risk of balloon rupture upon deployment.